1. Field of the Invention
This invention relates to polyisocyanate-modified dicarboxylic (poly)anhydrides, to a process for their production by reaction of carboxy-functional dicarboxylic (poly)anhydrides with organic polyisocyanates and to their use as curing agents for powder coating compositions based on polyepoxides.
2. Description of the Prior Art
Polyanhydrides of aliphatic dicarboxylic acids and their production from aliphatic dicarboxylic acids by reaction with acetic anhydride are known (Houben-Weyl, Methoden der organischen Chemie, 4. Ausgabe/Erweiterungs-und Folgebande, Band E 20, Part 2, pages 1400 to 1402). It is also known that corresponding polyanhydrides can be used as curing agents for powder-form epoxy resins (DE-OS 2,261,335 or BE-PS 819,623).
A polyol modification of these polyanhydrides which is particularly suitable for powder coatings is described, for example, in EP-A-0,299,420. A disadvantage of the paint systems specifically described in the examples lies in the comparatively high stoving temperatures of 163.degree. to 177.degree. C.
Although the systems according to EP-A-0,509,393, which also contain dicarboxylic polyanhydrides inter alia as curing agents, have comparatively lower stoving temperatures, they are dependent on the use of selected epoxy-functional copolymers having a high styrene content. The systems according to EP-A-0,544,206, which contain free dicarboxylic acids as curing agents, also have comparatively low stoving temperatures. Unfortunately, these systems are attended by the disadvantage of comparatively poor levelling properties, as shown in Comparison Example 7 of the examples.
It has now surprisingly been found that, by modifying carboxy-functional dicarboxylic (poly)anhydrides with polyisocyanates and optionally with compounds containing anhydride-reactive amino and/or hydroxyl groups, it is possible to obtain curing agents which, together with typical epoxy-functional paint resins, provide for the production of high-quality coating compositions. A particular advantage in this regard is that both the crosslink density and also the elasticity of the paints are increased with these curing agents. It is an additional advantage by using these compositions the stoving temperature can be reduced to 130.degree.-140.degree. C.